C8051F50x-F51x
23.4.2. SMB0CN Control Register
SMB0CN is used to control the interface and to provide status information (see SFR Definition 23.2). The
higher four bits of SMB0CN (MASTER, TXMODE, STA, and STO) form a status vector that can be used to
jump to service routines. MASTER indicates whether a device is the master or slave during the current
transfer. TXMODE indicates whether the device is transmitting or receiving data for the current byte.
STA and STO indicate that a START and/or STOP has been detected or generated since the last SMBus
interrupt. STA and STO are also used to generate START and STOP conditions when operating as a mas-
ter. Writing a 1 to STA will cause the SMBus interface to enter Master Mode and generate a START when
the bus becomes free (STA is not cleared by hardware after the START is generated). Writing a 1 to STO
while in Master Mode will cause the interface to generate a STOP and end the current transfer after the
next ACK cycle. If STO and STA are both set (while in Master Mode), a STOP followed by a START will be
generated.
As a receiver, writing the ACK bit defines the outgoing ACK value; as a transmitter, reading the ACK bit
indicates the value received during the last ACK cycle. ACKRQ is set each time a byte is received, indicat-
ing that an outgoing ACK value is needed. When ACKRQ is set, software should write the desired outgoing
value to the ACK bit before clearing SI. A NACK will be generated if software does not write the ACK bit
before clearing SI. SDA will reflect the defined ACK value immediately following a write to the ACK bit;
however SCL will remain low until SI is cleared. If a received slave address is not acknowledged, further
slave events will be ignored until the next START is detected.
The ARBLOST bit indicates that the interface has lost an arbitration. This may occur anytime the interface
is transmitting (master or slave). A lost arbitration while operating as a slave indicates a bus error condi-
tion. ARBLOST is cleared by hardware each time SI is cleared.
The SI bit (SMBus Interrupt Flag) is set at the beginning and end of each transfer, after each byte frame, or
when an arbitration is lost; see Table 23.3 for more details.
Important Note About the SI Bit: The SMBus interface is stalled while SI is set; thus SCL is held low, and
the bus is stalled until software clears SI.
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SILICON [ SILICON ]
